Bostons are much more easily trained than frenchies. They have less of a stubborn streak. Bostons have a bit of terrier in them and are much more playful/a bit more active(though this breed can be easily exercised in the house).

Strangely enough I have run into a number of dog aggressive frenchies. Most Bostons get along with everyone.

it sounds like you've made a good decision. Some dogs aren't cut out for SD work and it would be totally unfair to the dog if he is fearful.

The best pup for SD work is very brave and steady. You need one that takes new things in stride. You want a pup that gets mostly 3's and 4's on the Volhard puppy test and does a full or partial retrieve(a score of three or four on the retrieval test) at 7 weeks of age. Other than being steady and brave, the retrieval test is the single most important indicator of whether a dog can do SD work.